1. Field of the Invention
The present invention relates to a negative phase detecting circuit for a three-phase three-wire or three-phase four-wire type power supply source used as a power supply source for electronic equipment using a three-phase induction motor such as an air conditioner or the like.
2. Description of the Related Art
In a case where three-phase AC power is supplied to electronic equipment (for example, an air conditioner) using a three-phase induction motor (for example, a compressor of an air conditioner or the like), if the wiring between the three-phase AC power supply source and the electronic equipment is erroneous, there would occur such a disadvantage that the three-phase induction motor is rotated in the opposite direction to the expected direction or the like. Therefore, there has been proposed a negative phase detecting circuit for power supply for judging whether the wiring between a three-phase AC power supply source and electronic equipment is correct or not, and also whether the three-phase AC power supply source itself is proper or not (disclosed in Japanese Laid-open Patent Application No. Hei-8-289466).
FIG. 1 is a circuit diagram showing the construction of the power supply negative phase detecting circuit as described above.
According to the power supply negative phase detecting circuit shown in FIG. 1, two kinds of pulses, that is, a first pulse A (R-S) and a second pulse B (S-T) are generated from AC voltages of R-phase, S-phase and T-phase, and the phase difference between the first and second pulses is read out by a microcomputer to judge whether the phase is positive (correct wiring) or negative (erroneous wiring).
Further, there is known a low-cost outdoor unit of an only-cooling type air conditioner which is not equipped with any outdoor control board for controlling an outdoor unit. In this type low-cost outdoor unit, the control of the outdoor unit is carried out by the indoor side, and also a negative-phase preventing relay is provided to detect the negative phase and protect the outdoor unit under the negative-phase state.
However, since the internal parts of the negative-phase preventing relay are broken due to high voltage noises (thunder surge or the like) and thus a protecting device such as an arrester or the like must be separately equipped.
An object of the present invention is to perform negative phase detection by adding a simple electrical circuit to low-cost equipment and protect the electrical circuit from high voltage noises without mounting a varistor, an arrester or the like on a board, whereby the negative-phase detection, the protecting operation and the other functional performance can be supplied in a lower cost than when a negative-phase preventing relay is used.
In order to attain the above object, according to a first aspect of the present invention, there is provided a power supply negative-phase detecting circuit for electronic equipment using a three-phase AC power supply source, characterized by comprising: a pulse generator for generating a first pulse indicating the phase difference between any pair of two phases of three-phase AC power to be supplied to the electronic equipment, and a second pulse indicating the phase difference between another pair of two phases of the three-phase AC power, a differentiating circuit for generating a third pulse from the second pulse in synchronism with the rise-up of the second pulse so that the third pulse has a pulse width smaller than the phase difference between the first and second pulses; and a switching unit for subjecting the first pulse and the third pulse to AND operation and turning on/off the power supply to the electronic equipment on the basis of the AND operation result of the first and third pulses.
According to a second aspect of the present invention, in the power supply negative-phase detecting circuit of the first aspect of the present invention, when the three-phase AC power to be supplied to the electronic equipment has negative phase, the switching unit turns off the power supply to the electronic equipment.
According to a third aspect of the present invention, in the power supply negative-phase detecting circuit of the first aspect of the present invention, the differentiating circuit comprises a capacitor and a resistor.
According to a fourth aspect of the present invention, in the power supply negative-phase detecting circuit of the third aspect of the present invention, the switching unit comprises a first transistor having an emitter to which the first pulse is input, a second transistor having a base to which the third pulse is input, the base of said first transistor being connected to the collector of the second transistor to perform the AND operation between the first and third pulses, a smoothing capacitor which is charged in accordance with the AND operation result, a third transistor that is connected to the smoothing capacitor and turned on/off in accordance with the charging of said smoothing capacitor, and a relay switch for switching on/off the power supply to said electronic equipment in accordance with the ON/OFF operation of the third transistor.
According to a fifth aspect of the present invention, in the power supply negative-phase detecting circuit of the first aspect of the present invention, there is further provided alarm means for emitting an alarm when the three-phase AC power to be supplied to the electronic equipment has negative phase.
According to a sixth aspect of the present invention, in the power supply negative-phase detecting circuit of the fifth aspect of the present invention, when the three-phase AC power to be supplied to the electronic equipment has negative phase, the switching unit turns off the power supply to the electronic equipment, and simultaneously the alarm means emits the alarm.